A receiver intercepts signals and converts them into information for the listener. The information is transmitted in the form of modulation on an RF carrier wave. The receiver has a demodulator which removes the modulation components from the incoming signal. Circuitry then processes thes modulation components to provide useful information to the listener. There are situations where it is important to analyze the received signal as quickly as possible in order to evaluate whether the receiver is to process it further. In the past, the receiver included circuitry to look for the presence of the desired signal and when it was determined that it was indeed present, the receiver would be placed in condition for further processing of the signal. On the other hand, if the desired signal was not detected, it would not be further processed. This procedure of searching for the presence of the desired signal consumed too much time.
Two examples of receivers in which the search for the presence of a specific signal consumed too much time are those with battery saving and those that scan several channels.
The first type of receiver is portable and, therefore, incorporates batteries as the source of power. Throw-away batteries are less expensive and more convenient than rechargeable batteries which must be constantly recharged. However, throw-away batteries need to be replaced more often than rechargeable batteries. It has been proposed to increase the useful life and thereby improve the convenience of throw-away batteries by reducing the power consumed by the receiver. Battery-saver circuits deliver pulsed power to the various circuits in the receiver until a message for that receiver is received, at which time the power becomes continuous. Since it is during the pulses that the receiver is consuming power, it is desirable to minimize the pulse duration. However, each pulse must be long enough to insure that the signal will be detected and cause the battery saver circuit to provide continuous power.
Prior art detectors have not operated fast enough, particularly when the signal-to-noise ratio is poor. Also, the relatively slow speed of operation of prior art detectors which searched for signal presence necessitated excessively long battery-saver pulses. The basic problem with these prior art receivers is that they were pulsed on for a predetermined (fixed) time period.
Such battery-saver circuits may be employed in either a carrier-squelch receiver or in a selective call receiver. A carrier-squelch receiver is normally disabled so as not to reproduce noise or communications on the channel. Presence of a carrier wave, as indicated by the absence of noise from the demodulator, unsquelches the receiver. Such a receiver may have a battery saver circuit which produces pulsed power. In the presence of the carrier wave, the supply voltage becomes continuous. In order not to lose communications on the channel, it is desirable that the receiver rapidly respond when the carrier wave signal is present so that the battery saver pulses can be as narrow as possible. Because these prior art carrier-squelch receivers searched for the presence of the carrier wave (usually by detecting loss of noise from the demodulator), the battery-saver pulses were unnecessarily long thereby increasing power consumption by the receiver.
Such battery-saver circuits may also be employed in a selective-call receiver. A selective call communication system comprises a transmitter and a number of such receivers. Each receiver is designed to intercept the same carrier wave, but its alerting circuitry is rendered operative only when the carrier wave is modulated by a predetermined code. Upon reception of such code, the the receiver produces an audible or visible alerting signal. In certain kinds of systems, the signal is followed immediately by a voice message. In others, no voice message is transmitted. The possessor of the latter type, upon being alerted, will perform some previously agreed upon action such as calling his office. A selective-call receiver incorporates a code detector. Prior art detector circuits looked for the presence of a particular code. The code used to signal the receiver could be binary in nature or a tone signal. A tone signal code consists of a single tone, two or more simultaneous tones, or a tone sequence. In the case of a tone sequence, the detector maintains the supply voltage to the processor circuit continuous upon valid detection of the presence of the first tone. When the frequency of the tone perfectly matches the center frequency of the filter in the detector and the incoming signal is strong, detection takes place rapidly. The detector can produce an output very quickly to cause the battery-saver circuit to provide a continuous supply voltage, thereby keeping the processor circuit operative.
In prior art receivers the fact that detection of the signal is fast under such circumstances serves no useful benefit. This is because the processor circuit is operative for the duration of each battery saver pulse anyway, so that weak signals can be detected. When the receiver is far from the transmitter and is receiving a noisy carrier modulated with tone, the signal-to-noise (S/N) ratio is poorer, causing tone detection time to increase greatly. Thus, whether the detector detects the tone rapidly or not, the on period of the processor circuit must be long enough to detect the tone under all usable conditions.
Current is, of course, consumed for the duration of each pulse. The prior art receiver remains on and draws current for the entirety of each battery saver pulse, because it has been designed to hold the receiver on for detection of signal presence.
As mentioned above, the time for detection is also important in scanning receivers. In a scanning receiver, a local oscillator produces a locally generated signal which is applied at one input to a mixer, the other input of which is the incoming or RF signal. The mixer output, called a "high IF signal", has components at a frequency equal to the difference in the frequencies of the locally generated signal and the carrier wave. A low IF signal is generated by mixing the high IF signal with a second locally generated signal.
Channel selection is usually made by adjusting the frequency of the first locally generated signal. In a scanning receiver, variation in the frequency of the first locally generated signal is effected automatically. Detection of the presence of a carrier wave in the corresponding channel will lock the local oscillator onto the frequency at which it is then set, so that the carrier wave can be demodulated. This usually is accomplished by analyzing the output of the demodulator. If the noise content of such output exceeds a certain level, then the carrier wave on that particular channel is not present. If the noise is less than such level, a communication is taking place on the channel. In the past, scanning receivers had detectors that searched for the presence of the carrier wave. The longer it takes the detector to make its analysis, the longer the receiver must be tuned to each channel in order that it can be certain whether a communication is taking place on each channel.
When the incoming signal is strong, detection takes place rapidly. The detector can produce an output very quickly to cause the processor circuit to remain tuned to the channel then being scanned. However, the receiver may be a long distance from the transmitter so that the signal applied to the detector is mixed with noise; i.e. the signal is weak and the S/N ratio is poor. A poor S/N ratio causes the detection time to increase greatly. Thus, the detector responds rapidly in ideal situations but slowly when the S/N ratio is poor. Prior art scanning receivers had to be tuned to each channel for a predetermined (fixed) period of time long enough to enable the processor circuit to detect the presence of carrier under the worst S/N conditions.
As a result, fewer channels could be scanned in a given period of time.
Scanning receivers have been employed in selective-call communication systems. Each receiver in such a system is designed to intercept an RF signal with a carrier-wave frequency in one of a plurality of channels, but its alerting circuitry is rendered operative only when the carrier wave is modulated by a predetermined code. Upon reception of such code, the receiver produces an audible or visible alerting signal, as above described. Here, too, some past scanning receivers employed signal detectors which looked for the presence of the code. As a result, the receiver was required to scan each channel for a longer time in order that it could be certain whether a carrier wave on that channel was modulated by the code. The time on each channel could be no less than that required to detect the code's presence under the worst S/N ratio conditions.
Another reason that selective call scanning receivers in the past have not responded rapidly enough is because the filter(s) in the decoder retained some energy after analysis of the processed signal had been made. If that energy is not dissipated, the amplitude of the filter output will effectively be reduced by the energy left in the filter if the processor signal is not precisely in phase with the oscillations occurring in the filter by reason of such energy. Energy left in the filter, therefore, will lengthen the time the receiver must be set to a given channel, to insure detection.
A selective-call scanning receiver can incorporate battery saving. It is only during each pulse of power that the receiver is operative to scan all of the channels. The shorter the pulse duration, the less time the receiver will be set at a given channel. In a scanning receiver, therefore, it is doubly important that the detection time be as short as possible.
Filters used in selective-call receivers often include capacitors and inductors connected in series-resonant or parallel resonant circuits. However, inductors are expensive and generally not compatible with integrated circuits. Active filters have been used in the past to simulate inductors, but a filter of this type is a substantial drain on the battery.